A total of ten experiments have been performed during the tenure of this progress report, three basic and 7 DMBA tumor studies. The administration of corticosterone in the drinking water of intact rats blunted prolactin afternoon surges. Transplantation of adrenals to kidney capsule resulted in lower corticosterone levels but a diurnal rhythm in serum corticosterone was evident. Adrenalectomized (ADX) animals with dialysis sacs containing corticosterone survived a 30 day period in good health while animals bearing silastic capsules with a similar amount of corticosterone were dead by the end of the experimental period. Animals ADX at 45 days of age and placed on corticosterone in the drinking water had a delayed incidence of tumorigenesis with fewer tumors per animal. ADX at 40 days of age were comparable to controls in their pattern of tumorigenesis. The administration of PRL prior to and after DMBA administration hastened mammary tumor formation. There was no difference in the expression of mammary tumorigenesis when DMBA was injected in the morning vs the afternoon; however, in those animals given DMBA in the afternoon that were in vaginal proestrus, the duration to first tumor was delayed. Intact animals given corticosterone in the drinking water were observed to have tumors earlier than animals drinking tap water. Continued estrogen stimulation initiated either before or after DMBA administration blocked the incidence of mammary tumorigenesis.